An arrangement according to the third embodiment has already been set forth in the European patent document No. EP-A-0 152 026 (U.S.-A-4 649 322). In this arrangement triggering of the discharge in the lamp is brought about by a first generator which furnishes voltage pulses at predetermined periodic intervals. The luminous intensity of the lamp is controlled by a current source from a second generator which enables applying a discharge maintenance current to the lamp the duration of application of which may be varied according to the luminous intensity which one wishes to obtain. This arrangement further comprises a circuit which enables application of the maintenance current in synchronism with the voltage pulse.
In addition to two embodiments of the pulse generator, the cited document describes a manner of realizing the generator for maintaining discharge in the lamp. This maintenance generator which is a current source is energized from a DC voltage source and includes essentially two cascaded transistors which conduct continuously when an instruction signal is sent to the input of the first transistor. The duration of the application of the instruction signal (which may be a video signal for instance) determines the period during which the current source conducts, such period being on the order of 14 ms for a lamp giving full luminosity, and being followed by a series of periods of similar duration if the lamp must remain lighted at the full luminosity. In the case wherein the described arrangement is to be adapted in order to bring about simple variations of the luminous intensity of a fluorescent lighting fixture, for instance by means of a manual control, a single pulse furnished by a pulse generator at the moment of lighting of the fixture would be necessary, this pulse being followed by a continuous current maintained at the chosen level.
This manner of operation expends considerable electrical energy which is dissipated as heat and thus as a pure loss. Effectively, it is mentioned in the cited document that an energizing voltage of 60 volts DC enables assuring an arc voltage of about 40 volts in the tube, this leading one to understand that there is a voltage drop on the order of 20 volts which must be absorbed in the current generator. In reality one will note that the arc voltage may vary considerably (10 to 60 volts) depending in this respect on the dynamic program to which the lamp is subjected. The temperature has also an important influence on the value of this arc voltage. Thus, in the arrangement as cited, it is the current generator formed from the two transistors as hereinabove mentioned which must absorb the difference existing between the energizing voltage and the arc voltage, this difference being dissipated as a pure loss as has been mentioned.
The document US-A-3 890 537 describes a chopped energization acting as ballast for a gaseous discharge lamp.
In this document in order to energize the lamp there is employed a mains voltage source having its two alternations rectified. No filter has been provided following rectification. If the energizing system provides as is the case in the present invention, a chopping generator with a transistor and a diode, nevertheless the control of the current in the lamp is effected in a completely different manner to that set forth in the present invention in the sense that in the cited document, each time that the maximum current is attained, one turns off the transistor switch, this switch being turned on again when the minimum current is attained. There results from this a chopping frequency which is variable (between 10 and 40 KHz according to the text of the cited patent). In contrast thereto, the frequency of chopping of the present invention is fixed. If the cutoff of the transistor switch is brought about by a maximum current in the lamp, its reclosing on the other hand is independent of this current. There is thus no need in the present invention for a hysteresis comparator as is the case in the cited invention.
In the present invention thus, the lamp is energized from a DC voltage and from a chopping system having a fixed frequency. In the cited document, this voltage is not rectified and is not filtered and the chopping frequency is essentially variable. This cannot be suitable for energizing luminous points of a large matrix display board where it is necessary to control exactly the states of several neighbouring luminous sources.
It is thus the purpose of this invention to remedy the cited difficulties and to propose an arrangement which is a stabilized current source without itself consuming energy, whatever be the value of the load, such load being here manifested by the arc voltage presented by the lamp which is essentially variable.